197,307 research outputs found
The P-C phase transition of synthetic amphiboles in the NCMSH system: an FTIR study
No full textDuring the last years, a series of experimental studies have been done to characterize the structural features of synthetic amphiboles in the Li2O-Na2O-MgO-SiO2-H2O (LNMSH system) under different T, P and X conditions. Camára et al. (2003) and Iezzi et al. (2004) showed that the Li-free Na0.8(Na0.8Mg1.2)Mg5Si8O22(OH)2 compound is P21/m at room-T, and Camára et al. (2003) characterized the displacive P21/m C2/m phase-transition by single-crystal XRD. The phase transition of this amphibole has a macroscopic second order character, with a transition temperature (TC) of 257 °C.
Iezzi et al. (2005) did a similar investigation on the Na(LiMg)Mg5Si8O22(OH)2 composition by means of synchrotron radiation HT-XRPD, and showed that this compound also undergoes a displacive phase transition as a function of T, but the transition has a tricritical character and TC = 326 °C. Furthermore Iezzi et al. (2006) showed that all amphiboles in the LNMSH system, independently of their Na-Li contents at the B site, have the P21/m symmetry at room conditions.
Natural Group 5 amphiboles which belong to the B(Na,Li)2, B(Na,Mn)2 and B(Ca,Mn)2 solid-solutions, whether their A site is filled or empty, have monoclinic C2/m symmetry. In contrast, the A-site filled synthetic samples belonging to the B(Na,Mg)2 and B(Li,Mg)2 solid-solutions, have P21/m symmetry at room T, and undergo phase-transitions at variable TC, depending on their chemical composition (Welch et al. 2007). The P21/m symmetry is restricted to amphiboles with a double-chain composition close to Si8 and is probably stabilized by small cations only at the B-group sites or by a B compositions where large monovalent cation (Na) and a small divalent cation (Mg) have a ratio close to 1:1. However, the dimension of the C-group cations also play a significant role in determining the transition behavior (cf. Welch et al. 2007 for more discussion).
In this work, we explore the effects on the phase transitions of a large divalent cation as Ca which progressively replaces BMg. The amphibole symmetry was monitored using in situ low- and high-T FTIR spectroscopy. Samples were prepared as conventional KBr disks (Iezzi et al., 2006); low-T spectra were collected using a nitrogen-cooled cryostat, while high-T spectra were collected using a Specac microfurnace. Previous work (Iezzi et al., 2004, 2006) show that OH-stretching FTIR spectroscopy is particularly useful in characterizing the symmetry changes associated with the P C transition in these amphiboles. For varying T conditions there are well-defined modifications of the room-T pattern, consisting of progressive band broadening and band splitting. Inspection of the data shows that for increasing BCa in the amphibole structure there is a linear decrease of the transition temperature TC for the Mg-richer samples, as also evidenced by the companion X-Ray diffraction study (Iezzi et al., this meeting).
References
Cámara F., Oberti R., Iezzi G., Della Ventura G. (2003) Phys. Chem. Miner., 30, 570-581.
Della Ventura G., Iezzi G., Bellatreccia F., Càmara F., Oberti R. (2004). Mitt. Oster. Min. Ges., 149: 23.
Iezzi G., Della Ventura G., Oberti R., Cámara F., Holtz F. (2004) Am. Miner., 89, 640–646.
Iezzi G., Tribaudino M., Della Ventura G., Nestola G., Bellatreccia F. (2005) Phys. Chem. Miner., 32, 515–523.
Iezzi G., Della Ventura G., Tribaudino M. (2006) Am. Miner., 91, 425-429.
Welch M.D., Cámara F., Della Ventura G., Iezzi G. (2007) Rev. Mineral. Geochem., 67, in press
Synthetic P21/m amphiboles in the system Li2O-Na2O-MgO-SiO2-H2O (LNMSH)
No full textWe describe here the synthesis of amphiboles along the nominal Na(NaMg)Mg5Si8O22(OH)(2)- Na(LiMg)Mg5Si8O22(OH)(2) join, at 800 degrees C, 0.4 GPa. High amphibole yields (> 90%) plus minor quartz and enstatite have been obtained at all compositions; amphibole crystals are acicular and their size rarely exceeds 20-30 X 0.5-3 mu m. TEM analysis shows the presence of h+k odd reflections in all samples, indicative of a P-lattice. By similarity with closely related amphiboles from the literature (c,g., Oberti et al. 2000; Camara et al. 2003) it P2(1)/m space group was assigned to the amphiboles synthesized here. Refined cell-parameters from X-ray powder-patterns show a linear decrease as a function of increasing Li at M4, a and P being the most affected parameters. The four infrared OH-stretching spectra all show two main bands at 3741-3748 and 3712-3716 cm(-1), respectively. They are assigned to two independent O-H groups in the P2(1)/m structure, interacting with a strongly delocalized Na-A. The spectra show in addition two minor absorptions at about 3688 and 3667 cm(-1), respectively; these bands are assigned to vacant A-sites in the structure and indicate slight departure of the nominal composition toward cummingtonite. The present work shows that one apfu of Na-B can also be completely replaced by one apfu of Li-B (M+), in synthetic Na(M+Mg)Mg5Si8O22(OH)(2) and that all compositions have P2(1)/m symmetry at ambient conditions
Synchrotron infrared spectroscopy of synthetic Na(NaMg)Mg(5)Si(8)O(22)(OH)(2) up to 30 GPa: Insight on a new high-pressure amphibole polymorph
No full textThis paper describes a high-pressure synchrotron infrared (IR) Spectroscopy Study of the synthetic amphibole Na(NaMg)Mg(5)Si(9)O(22)(OH)(2). This compound has P2(1)/m symmetry at room conditions; its IR OH-stretching spectrum Consists Of two main bands at 3743 and 3715 cm(-1), which are assigned to the two symmetrically independent O-H groups in the P Structure (sample 403, Iezzi et al. 2004a). For increasing pressure, both bands shift toward higher frequency, suggesting a shortening of the O-H bond. In addition, the two bands progressively merge to give a single, symmetric and broad absorption band at 20-22 GPa. This behavior suggests that at 20-22 GPa there is a unique O-H group in the structure, indicative of a C-lattice type. The IR data thus show that the examined sample undergoes a P2(1)/m C2/m phase-transition at that pressure. Upon release of pressure, the initial two-band pattern is immediately recovered indicating that the pressure-induced phase-transition is reversible, as already observed for the same kind of transition induced by temperature. By analogy with structurally related pyroxenes, and taking into account the possible crystal Structural topologies of amphiboles, we suggest that the C2/m polymorph stable at high pressure is characterized by fully kinked double-chains
Synthetic ANaB(NaxLi1 ¡ xMg1)CMg5Si8O22(OH)2 (with x = 0.6, 0.2 and 0) P21/m amphiboles at high pressure: a synchrotron infrared study
No full textThe high-pressure behavior of three synthetic amphiboles crystallized with space group P2(1)/m at room conditions in the system Li(2)O-Na(2)O-MgO-SiO(2)-H(2)O has been studied by in situ synchrotron infrared absorption spectroscopy. The amphiboles have compositions (A)Na (B)(Na (x) Li(1 - x) Mg(1)) (C)Mg(5) Si(8) O(22)(OH)(2) with x = 0.6, 0.2 and 0.0, respectively. The high-P experiments up to 32 GPa were carried out on the U2A beamline at Brookhaven National Laboratory (NY, USA) using a diamond anvil cell under non-hydrostatic or quasi-hydrostatic conditions. The two most intense absorption bands in the OH-stretching infrared spectra can be assigned to two non-equivalent O-H dipoles in the P2(1)/m structure, bonded to the same local environment (M1M3)Mg(3)-OH-(A)Na, and pointing toward two differently kinked tetrahedral rings. In all samples these bands progressively merge to give a unique symmetrical absorption with increasing pressure, suggesting a change in symmetry from P2(1)/m to C2/m. The pressure at which the transition occurs appears to be linearly correlated to the aggregate B-site dimension. The infrared spectra collected for amphibole (B)(Na(0.2)Li(0.8)Mg(1)) in the frequency range 50 to 1,400 cm(-1) also show a series of changes with increasing pressure. The data reported here support the inference of Iezzi et al. (Am Miner 91:479-482, 2006a) regarding a new high-pressure amphibole polymorph
M - Il mostro di Düsseldorf. Per un’idea di giustizia
No full textIl volume raccoglie contributi che vanno da alcune analisi sul cinema tedesco – dall'espressionismo alla propaganda di Leni Riefenstahl – al rapporto tra tecnica ed espressione; al lavoro di un musicista, Mario Castelnuovo-Tedesco; allo scontro tra Pier Paolo Pasolini e la censura italiana
M - Il mostro di Düsseldorf. Per un’idea di giustizia
No full textIl volume raccoglie contributi che vanno da alcune analisi sul cinema tedesco – dall'espressionismo alla propaganda di Leni Riefenstahl – al rapporto tra tecnica ed espressione; al lavoro di un musicista, Mario Castelnuovo-Tedesco; allo scontro tra Pier Paolo Pasolini e la censura italiana
Social Media Analytics e creazione di conoscenza: le tappe di un’analisi sperimentale su Twitter
No full textText analytics: present, past and future
No full textText analytics is a large umbrella under which it is possible to report countless techniques, models, methods for automatic and quantitative analysis of textual data. Its development can be traced back the introduction of the computer, but the prodromes date back, the importance of text analysis has grown over time and has been greatly enriched with the spread of the Internet and social media, which constitute an important flow of information also in support of official statistics. This paper aims to describe, through a timeline the past, the present and the possible future scenario of text analysis. Moreover, the main macro-steps for a practical study are illustrated
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